If you can find it on GitHub and Stack Exchange, it’s not plagiarism

As a software developer I’m expected to, at the very least, to do two things when “plagiarizing”:

1. Find the source to copy from.
2. Perform the necessary adjustments to apply that copied solution to my own problem.

When students plagiarize, they don’t even need to do that. The solution they are copying from was written for the exact same assignment, so they don’t need the adjust anything (at most, they change some identifiers to throw off plagiarism detectors). And they copy from each other, so they don’t need to search for a solution. They may need to apply some social skills to find out who to copy from - but these are vastly different from the technical skills required to find relevant code to “plagiarize” in real world programming.

those programmers understand the underlying code.

This is about students PROVING they understand the underlying theory. Letting them copy destroys that.